Russian MPs who have been calling for Crimea to be transferred to the Russian Federation should be included in any sanctions imposed by the EU over the crisis in Ukraine, David Cameron has said.

EU leaders warned last week that the 28-nation bloc will implement travel bans and asset freezes if Russia fails to engage with Ukraine and join an international "contact group" of states seeking to resolve the current crisis.

Downing Street had previously indicated that the measures could target "officials" in the administration of President Vladimir Putin, but Mr Cameron has made clear that he also wants any travel ban to include MPs in Russia's Duma who have been vocal in support of what he regards as violations of Ukraine's sovereignty.

The Prime Minister declined to name individuals or put a figure on the number of MPs he wants to target. But prime candidates could include 15-20 MPs who have put their names to legislation, due to be debated in the Duma on Monday, which would legitimise the result of Sunday's referendum in Crimea on whether the peninsula should remain part of Ukraine or join Russia.

Mr Cameron has said the planned referendum would be illegal and has made clear that any move by Russia officially to recognise its outcome would be regarded as a further escalation of the crisis.

EU foreign ministers, including William Hague, will meet in Brussels on Monday, when a decision could be taken to impose further sanctions, which could be in place within two or three days.

"We will be pushing for travel bans to include some prominent Russian MPs," the Prime Minister told reporters on his flight to Israel, where he is making a two-day visit.

"I am not naming names today. The criteria is people who have been pushing for the unacceptable steps that have been taken."

It is not thought that Mr Putin himself or senior members of his administration, like Foreign Minister Sergey Lavrov, are likely to be included in a first round of EU sanctions.

But Mr Cameron confirmed that the possibility of including MPs was discussed at a meeting in London yesterday of officials from EU states and other international powers such as the US, Japan, Canada, Switzerland and Turkey.

Leaders of the 28 EU member states met in Brussels last week to give approval to a co-ordinated three-stage ratcheting-up of pressure on the Kremlin.

Having already withdrawn co-operation with Moscow on a range of issues as an immediate response to Russia's effective military takeover of Crimea at the end of last month, the meeting agreed that a second phase could include travel bans and asset freezes if the Putin regime refuses to talk to Ukraine or engage with the contact group, with wider economic sanctions held back for a third phase if the situation escalates.

Mr Cameron admitted that the readiness of all EU states to agree on "tough and important" measures surprised some people.

"Everyone was expecting that the US would take a series of steps and the EU would fall short," he said. "Britain played an important role bringing countries together, bringing together the Poles - who, like us, want to see strong action - and other countries, forging consensus to find a way through this.

"The three-phase process is very sensible. It sets Europe on a pathway and will make subsequent meetings - and I suspect there will be subsequent meetings of foreign ministers and possibly heads of state and government - on a path of triggering those phases as and when it is necessary."

In a broadcast interview shortly after his arrival in Israel, Mr Cameron said: "The European Council has been clear if this Contact Group, if this talks process between Ukraine and Russia doesn't get going and doesn't make progress, there should be asset freezes and travel bans.

"Britain will be pushing for some of these travel bans to be applied to some of these Russian Members of Parliament who are partly responsible for the situation that people of Ukraine face today."

Mr Cameron, along with his fellow G7 leaders, issued a statement saying they would not recognise the outcome of the Crimean referendum and warned of "further action" if Russia annexed the region.

The leaders c alled on Moscow to "cease all efforts to change the status of Crimea contrary to Ukrainian law and in violation of international law".

They went on: "We call on the Russian Federation to immediately halt actions supporting a referendum on the territory of Crimea regarding its status, in direct violation of the constitution of Ukraine.

"Any such referendum would have no legal effect. Given the lack of adequate preparation and the intimidating presence of Russian troops, it would also be a deeply flawed process which would have no moral force.

"For all these reasons, we would not recognise the outcome.

"Russian annexation of Crimea would be a clear violation of the United Nations Charter; Russia's commitments under the Helsinki Final Act; its obligations to Ukraine under its 1997 Treaty of Friendship, Cooperation and Partnership; the Russia-Ukraine 1997 basing agreement; and its commitments in the Budapest Memorandum of 1994.

"In addition to its impact on the unity, sovereignty and territorial integrity of Ukraine, the annexation of Crimea could have grave implications for the legal order that protects the unity and sovereignty of all states.

"Should the Russian Federation take such a step, we will take further action, individually and collectively."

The G7 statement went on: "We call on the Russian Federation to de-escalate the conflict in Crimea and other parts of Ukraine immediately, withdraw its forces back to their pre-crisis numbers and garrisons, begin direct discussions with the government of Ukraine, and avail itself of international mediation and observation offers to address any legitimate concerns it may have.

"We, the leaders of the G7, urge Russia to join us in working together through diplomatic processes to resolve the current crisis and support progress for a sovereign independent, inclusive and united Ukraine.

"We also remind the Russian Federation of our decision to suspend participation in any activities related to preparation of a G8 Sochi meeting until it changes course and the environment comes back to where the G8 is able to have a meaningful discussion."

US secretary of state John Kerry has announced he will meet Mr Lavrov in London on Friday for talks ahead of the Crimea referendum.

Downing Street said the Foreign Secretary suggested the UK could be used as a meeting place to his US counterpart earlier this week.

The Prime Minister's official spokesman said: "It was something the Foreign Secretary, I think, had mentioned to Secretary Kerry a few days ago, that if London could be a place for future discussions between Russia and the United States as part of the international discussions, then they would be very welcome."

He added: "We, and others, have said we want to continue talking to the Russian authorities... but what is critical here in terms of the key next step is for Russia to be talking to the Ukrainian government. That remains, as per the G7 statement issued today, the key thing that needs to happen."

America's ambassador to the UK Matthew Barzun tweeted: " I am pleased that London will host the JohnKerry and mfa_russia FM Lavrov meeting this Friday on Ukraine. Dialogue is the way forward."